Series connected battery stacks are being utilized in many applications such as telecommunication power supplies, electric vehicles (EVs), uninterruptible power supplies (UPS) and photovoltaic (PV) systems. Battery life is one of the major factors presently limiting the realization of economical systems. Series connected battery strings are prone to dramatic reduction in life and potential damage if high rate charging is continued after the onset of gassing. Differences in cell chemistry, and normal differences during repeated cycles of cell charge and discharge, lead to large non-uniformities in cell charge levels and correspondingly different cell terminal voltages. During charging of a battery composed of series connected cells, some cells will consequently reach full charge before others and before the overall battery terminal voltage reaches its nominal value. Such a process leads to overcharging of a subset of cells. If these cells are charged into the gassing phase, there can be significant degradation of the battery life. Maintenance of cells at an equalized charge level is critical for enhancing battery life. The same is true for strings of batteries connected together. For convenience, the term "battery" as used hereafter may refer to either a single cell or to a unit composed of internally or externally connected cells.
Different schemes and algorithms have been developed to achieve battery charge equalization. One approach which has been widely applied to minimize irregularities among battery modules has been regular charging at raised voltage levels. In this case, all batteries are further charged until they can be safely assumed to be fully charged regardless of their actual state of charge. This can adversely affect the lifetime of the batteries in addition to increasing the power consumption within the battery stack. Finally, the battery utilization is reduced due to the large difference between the working and discharge voltages.
Another algorithm uses a multi-step constant current charging. See, C. C. Chan, et al., "A Microprocessor Based Intelligent Charger for Electric Vehicle Lead Acid Batteries," Electric Vehicle Symposium, EVS-10, pp. 456-466, Hong Kong, 1990. This charging process starts with the maximum current the charger can deliver to the battery pack at low state of charge. As the state of charge builds up, the charging current is reduced in steps. Rest (cooling) periods are also incorporated in the algorithm to minimize temperature differences between the cold and warm batteries. Pulse equalization charging is applied at low current levels to improve the battery capacity. The batteries are normally maintained at the fully charged state before use using pulse trickle charging.
Other schemes and algorithms have been reported as well. However, since all of these schemes deal with the battery pack as a whole, individual battery (cell) equalization cannot be easily achieved. To prevent the adverse effects of unequalized charging of batteries, individual batteries need to be maintained at an equalized charge level. The fact that individual batteries should have the same voltage level once they have reached the final state of charge can be utilized to achieve this task.
A simple approach to equalize the batteries in a stack is to use bypass resistive shunts across each individual battery. The amount of current drawn by the shunt elements is proportional to the individual battery voltage, which results in more current being diverted to the shunt as the battery voltage increases. This will tend to reduce the voltage differences between the different batteries within the stack since higher voltage batteries will be further discharged by the shunt elements. One drawback of this approach is that the recovered energy is being converted into additional losses in the shunt elements. In addition, the amount of current drawn by the shunt elements is not regulated. As a result, the battery voltages are not fully regulated.
To regulate the current drawn by the shunt elements, active circuitry can be used. The use of individual cell equalizers (ICE) is one such approach. See, D. Bjork, "Maintenance of Batteries--new trends in batteries and automatic battery charging," INTELEC Conf. Proceedings, pp. 355-360, 1986; S. Bergvik, "Prolonged Useful Life and Reduced Maintenance of Lead-Acid Batteries by Means of Individual Cell Voltage Regulation," INTELEC Conf. Proceedings, pp. 63-66, 1984. The ICE is a voltage controlled current shunt which diverts the current away from the cell during trickle charging. This scheme prevents fully charged cells from getting overcharged while undercharged modules can still be trickle charged. In this scheme, the amount of lost energy is minimized since the shunt circuitry is only active when the cell voltage exceeds the preset level.
The ICE scheme is best suited for low charge/discharge current levels (0.1%-1%). In these applications, the active devices can be signal level devices where the maximum shunt current is on the order of few hundred milliamps, which limits the energy dissipated in the shunt resistors.
In the above mentioned schemes, the recovered energy is dissipated into the resistive current shunts. As a result, these dissipative equalization schemes are best suited for low power applications and/or low current charge/discharge rates to minimize the lost energy.
In applications such as electric vehicles (EVs), the current charge/discharge rates are relatively high (10%-100%), and the charge/discharge times are quite short. As a result, the charge equalization currents could be of the same order of magnitude as the charge and discharge currents. Thus, to minimize energy loss and optimize performance of the batteries, non-dissipative equalization techniques must be used.
One approach to achieving charge equalization at high charging rates utilizes isolated dc-to-dc converter modules across each battery cell. An example of this approach is a fly-back dc-to-dc converter connected across each battery module. This approach is best suited for large battery systems where a group of batteries have a dedicated dc-dc converter. When the voltage of a given battery module exceeds a preset voltage level, the excess energy is transferred back to the battery bus, thus allowing the battery voltage to be precisely regulated. The control signals for the power semiconductor switches are derived via simple comparative circuitry which detects the deviation of the module voltage from the average preset value. An isolation transformer is required for each series battery to provide the required level shifting.
A variation of this approach involves using bi-directional dc-to-dc converters across each battery cell/module is discussed in H. Schmidt, et al., "The Charge equalizer--A New System to Extend Battery Lifetime in Photovolatic Systems, U.P.S. and Electric Vehicles," INTELEC Conf. Proceedings, pp. 146-151, 1993. Using this approach, energy can be transferred from each battery module to the main battery bus and vice versa. During charging, the voltages of individual batteries are regulated by transferring the excess energy to the battery bus. During operation (discharging), energy can be transferred from the battery stack to weak batteries, thus maintaining all batteries at the same level and improving the utilization of the battery stack. Similar to the previous approach, the driving signals for the semiconductor switches are derived using simple comparative circuitry which activates the appropriate side of the converter.
The problem with these approaches is that a dedicated dc-to-dc converter is used for each battery module. In addition, the dc-dc converter and the transformer winding have to be rated for the full dc bus voltage. As a result, such equalizers must be custom designed and cannot be modularized for use with different battery bus voltages.
In the previously described schemes, a dedicated dc-to-dc converter across each battery is used. As a result, the number of active switches and hence the number of control circuit elements is quite high. To minimize the required hardware, a centralized equalizing converter with a multi-winding transformer can be used. The simple implementation can be realized via the use of a single isolated dc-to-dc fly-back converter with multi secondary windings as described in H. Schmidt, et al., "The Charge equalizer--A New System to Extend Battery Lifetime in Photovolatic Systems, U.P.S. and Electric Vehicles, " INTELEC Conf. Proceedings, pp. 146-151, 1993. The primary side is connected to the battery bus while each of the secondary windings is connected across each battery or group of batteries. Consequently, the number of secondary windings is the same as the number of batteries. In this approach, the battery voltages are precisely regulated by transferring energy from the battery bus to the weakest battery within the stack. Ideally, the converter is idle until a weak battery is detected. When the switch is closed, energy is stored in the magnetizing inductance of the transformer, which is later transferred to the low voltage batteries upon the switch turn-off. Note that all the coils are wound on a common core which dictates that all voltages will be the same. Ideally, the largest portion of the stored energy will be directed to the lowest voltage module without any additional control. In reality, this scheme has fairly high sensitivity to the leakage inductance between secondary windings.
Another approach which utilizes a centralized equalizing converter is the forward converter with a multi-winding transformer. A two switch forward converter can be used. See, N. H. Kutkut, et al.; "Charge Equalization for series Connected Battery Strings," IEEE IAS Annual Meeting, October 1994, pp. 1008-1015; N. H. Kutkut, et al., "Design Considerations for Charge Equalization of an Electric vehicle Battery System," IEEE APEC Conf. Rec., pp. 96-103, 1995; D. M. Divan, et al., "Battery Charging Using a Transformer with a Single Primary Winding and Plural Secondary Windings", U.S. Pat. No. 5,659,237, 1997. The voltage source voltage can be the same as the battery bus voltage. When a low voltage battery is detected, the converter is activated to transfer energy from the overall battery stack to the weakest battery. The individual battery voltages are regulated by controlling the duty cycle of the power MOSFETs.
Other approaches rely on utilizing a centralized multi-winding transformer. The equalizer/diverter circuit consists of isolated fly-back converter modules across each battery. The transformer coils are tightly coupled and are wound on a common core. In addition, the turns ratio is nearly unity. If a battery becomes overcharged, the corresponding switch is turned on, thus storing energy in the magnetizing inductance of the transformer. When the active switch is turned off, the stored energy is distributed among all batteries with most of the energy directed to the weakest battery. This allows direct energy transfer from healthy batteries to weak batteries during both charging and discharging. If more than one battery becomes overcharged, the battery with the highest voltage will dominate and feed energy to the weakest battery within the stack.
The number of windings can be further reduced if a forward type converter is used. A modified current diverter with a centralized multi-winding transformer with forward converter modules can be used. In this case, the number of secondary windings is the same as the number of battery modules. In addition, no rectifying diodes are required since the MOSFETs' anti-parallel body diodes are used for rectification.
Implementing any of the foregoing schemes for large series connected battery banks is rather tedious because a transformer with multiple windings is needed, and interconnecting these windings to the different batteries could become quite complex. In addition, these approaches cannot be modularized because the number of secondary windings has to match the number of batteries or group of batteries.